1. Field of the Invention
The present invention relates to an optical apparatus for transmitting data between two members whose positions are changed relative to each other, for example, an optical data transmission apparatus for transmitting data between a rotating portion and a fixed portion in a gantry of an X-ray computerized tomography (CT) scanner.
2. Description of the Related Art
Generally, in an X-ray CT scanner, data transmission between the rotating portion and the fixed portion in the gantry is effected via a slip ring. That is, a conductive metal is arranged in a ring form on the rotating portion side, a brush formed of a conductive member is disposed on the fixed portion side, and the brush is always set in contact with the ring irrespective of the rotation/stoppage of the rotating portion. Contact between the brush and the ring causes the rotating portion and the fixed portion to be electrically connected to each other, thereby making it possible to permit data to be transmitted between the rotating portion and the fixed portion during the rotation.
However, in the above slip ring system, since the brush is always set in mechanical contact with the ring, wear would occur in the mechanical contact portions thereof. Therefore, periodic maintenance is required and it is troublesome. Further, data transmission is sometimes instantly stopped (instantaneously interrupted) by the poor contact or contact resistance between the ring and the brush, thus lowering the reliability and data transmission quality. The amount of data to be transmitted is dependent on the number of rings and the ability (rate) of data transmission between the ring and the brush and the rate is limited. Therefore, in order to increase the amount of transmission data to exceed the limited value, the number of rings must be increased, and in this case, the size of the mechanism becomes large.
In order to overcome the above problems, an optical data transmission apparatus is recently developed as recited in U.S. Pat. No. 4,996,435 (Keller). This apparatus comprises plural light emitting elements disposed on the rotating portion side and a light receiving element disposed on the fixed portion side and faced toward the light emitting elements for receiving light emitted from at least one light emitting element. Since the light is emitted in a plane perpendicular to the axis of the patient or the rotating axis of the rotating portion, manufacturing of the apparatus is difficult. Surfaces of the rotating portion and the fixed portion facing each other are cylindrical surfaces. Therefore, it is difficult to dispose the light emitting elements and the light receiving element on the cylindrical surface.
The number of the data channel is one in this prior art. Plural items of data cannot be simultaneously transmitted. Therefore, it is difficult to effectively transmit a large amount of data.
Further, the distance between the light emitting element and the light receiving element periodically changes during the rotation of the rotating portion. Thus, the amount of light received by the light receiving element also periodically changes during the rotation. Therefore, it is necessary to increase the dynamic range of a receiver circuit in order to cope with this variation. However, the S/N ratio is decreased when the dynamic range is increased.